Recently, interests in the energy storage technology are increasing gradually. As the application areas expand toward mobile phones, camcorders, notebook PCs and electric vehicles, efforts for research and development of electrochemical devices are being materialized more and more.
The electrochemical devices are drawing attentions the most in this aspect. In particular, the focus of development is on rechargeable secondary batteries. Recently, with regard to the development of secondary batteries, research and development are being carried out actively on the design of new electrodes and batteries for improvement of capacity density and specific energy.
Among the currently used secondary batteries, the lithium secondary battery developed in the early 1990s is appreciated due to the advantages of higher operating voltage and outstandingly hither energy density as compared to the conventional batteries using aqueous electrolytes, such as Ni-MH, Ni—Cd or sulfuric acid-lead batteries.
A separator used in the lithium secondary battery serves to physically block the contact between a positive electrode and a negative electrode while allowing the transport of lithium ions and to electrically insulate the positive electrode and the negative electrode. Particularly, the separator is known to have a significant effect on the characteristics and safety of the battery because it maintains insulation for preventing internal short circuits.
Hitherto, the dielectric breakdown voltage of the separator itself was measured in order to evaluate the insulating performance of the separator. Specifically, after sandwiching the separator between an upper jig and a lower jig and applying a voltage between the two jigs, the voltage at which a current above a reference value flew through the separator was measured as the dielectric breakdown voltage.
However, only the insulating performance of the separator itself could be measured and the dielectric breakdown voltage of an electrode assembly wherein the separator is laminated with electrodes could not be measured. The separator contained in the electrochemical device is laminated with the electrodes during the assemblage of the electrode assembly. This lamination includes a process of laminating the separator with the electrodes coated with active material particle layers and then compressing the same using a roller. The dielectric breakdown voltage is different before and after the lamination because the separator is deformed during this process. It is because fine unevenness is formed on the surface of the separator while the active material particles present on the electrode surface and the separator are compressed, leading to variation in thickness throughout the surface. In addition, if the electrode active material particles are deintercalated or foreign impurities such as metal dust, etc. are interposed between the separator and the electrodes during the lamination process, the dielectric breakdown voltage may be changed. In this case, the insulating performance worsens as the thickness of the region of the separator contacting with the impurities is decreased. It is because resistance is decreased locally at the region with the decreased thickness. However, with the existing method, the difference of the dielectric breakdown voltage of the separator itself and the dielectric breakdown voltage of the separator after the lamination could not be measured.
Meanwhile, Korean Patent Publication No. 10-2016-0102331 discloses a method of interposing a sheet of negative electrode and a sheet of separator between two jigs and measuring dielectric breakdown voltage by measuring the resistance when a nail passes therethrough. However, with this method, it is difficult to determine whether the dielectric breakdown voltage is due to the separator or due to the electrode.